In the process of developing imagewise exposed photographic silver halide emulsions, a combination of special emulsions and developers is required to give the high gradient, sharp toe, low fog and high top density characteristic of graphic arts products. Developers conventionally used in this process are based on hydroquinone as taught by Mason in "Photographic Processing Chemistry," Focal Press, pages 165-170 (1975). This combination of emulsion-developer is used mainly in the production of half-tone dot images and high contrast line images, conventionally used in letterpress, lithography and the like. It is known that these developers exhibit an induction period prior to development of the exposed sliver halide, whereafter so-called "infectious development" occurs giving rise to the high gradients necessary to produce useful images for graphic arts processes.
To produce high gradient in a half-tone dot, maximum density must be developed within the dot, but with little, if any, development in the unexposed emulsion surrounding the dot. Good dot quality is a combination of these factors, as pointed out by Mason:
(a) edge of dot must be sharp, PA1 (b) little or no fog between dots, PA1 (c) high dot density, PA1 (d) no "halo" around the dots, PA1 (e) infinite density gradient across the dot edge, PA1 (f) optimum quality at normal processing conditions. PA1 (1) Hydroquinone or a substituted hydroquinone in combination with at least one other superadditive developing agent,
In order to enhance the infectious development which provides this dot quality, these developers contain an unusually low concentration of alkali sulfite. Since a low sulfite level renders a developer more prone to aerial oxidation, it is common to add carbonyl bisulfite compounds to act as a reservoir of sulfite. However, formaldehyde, which results from the dissociation of carbonyl bisulfite, reacts in a complex manner with the semiquinone, which is in equilibrium with the hydroquinone, which further tends to degrade the activity of the developer solution. These reactions are described by Childers, Photographic Science and Engineering, Vol. 15, page 480 (1971).
With the advent of processing machines and, more particularly, rapid access processing machines, the deficiencies of lithographic processing chemistry became more serious and apparent. Developer degradation was accelerated under machine processing conditions. The lag in the start of development caused by the long induction period of hydroquinone developers lengthened the processing time and delayed access to the finished product. While the induction period could have been eliminated, and the developing process speeded up, by using so-called "rapid access" developers containing both hydroquinone and an auxiliary developing agent such as phenidone or metol, the trouble is that these auxiliary developing agents are not useful in lithographic development because they cannot produce the necessary high gradient. The reason for this is that rapid access developers have a high sulfite content and this reacts destructively with semiquinone to prevent infectious development. Therefore, the relatively high level of sulfite in rapid access developers results in lower contrast than lith developers. It would obviously be desirable to combine the high gradient of lith developers with the processing convenience and stability of the rapid access developers.